Method and system for measuring the imaging quality of an optical imaging system

ABSTRACT

An object pattern is imaged by an imaging system onto the image plane of the imaging system at a location where a reference pattern suited to the object pattern is situated in order to measure the imaging fidelity of an optical imaging system, for example, an eyeglass lens, a photographic lens, or a projection lens, for use in the visible spectral range. The resultant, two-dimensional, superposition pattern is detected in a spatially resolved manner in order to determine imaging parameters therefrom. The object pattern is generated with the aid of at least one electronically controllable pattern generator that serves as a self-luminous, electronically configurable, incoherent light source and may, for example, have a color monitor. The measuring system allows rapidly, flexibly, checking optical imaging systems with minimal time and effort spent on the mechanical setup required.

[0001] The following disclosure is based on German Patent ApplicationNo. 10154125.2, filed on Oct. 25, 2001, which is incorporated into thisapplication by reference.

FIELD OF AND BACKGROUND OF THE INVENTION

[0002] The invention relates to a measuring method and measuring systemfor measuring the imaging fidelity of an optical imaging system.

[0003] Optical imaging systems are employed in numerous fields ofengineering and research that impose increasingly stringent demands ontheir imaging fidelity. An example of such a field is photolithographicfabrication of semiconductor devices and other types of microdevices,under which submicrometer-scale structures are created usinghigh-performance projection lenses. Another example is photographiclenses of all types, which are usually subject to less stringent demandson their imaging fidelities.

[0004] Imaging optics frequently have elaborate layouts involvingnumerous lenses, which usually makes it impossible to derive theiroptical properties from theoretical computations. The optical propertiesof imaging systems thus must be reliably measured, where the accuracy ofthe method employed for testing the imaging systems involved is normallyadapted to suit the demands imposed on their imaging accuracies.

[0005] Interferometric measurement methods are frequently employed. Adevice operating similarly to a shearing interferometer that allowsmaking rapid, high-precision, measurements on photolithographicprojection lenses is described in the German patent application havingthe filing code DE 101 09 929.0. In the case of that device, a maskilluminated by incoherent light is arranged in the object plane of theimaging system to be tested. The mask comprises a rigid, transparent,substrate fabricated from, for example, quartz glass, to which atwo-dimensional object pattern is applied by, for example, suitablecoating it with chromium. A reference pattern configured in the form ofa diffraction grating is arranged in the image plane of the imagingsystem. Superimposing the waves created by diffraction on one anothergenerates a superposition pattern in the form of an interferogram thatis detected with the aid of a suitable (spatially resolving) detector.

[0006] Several interferograms having differing phases are required inorder to compute two-dimensional phase distributions from theseinterferograms. Their phase may be varied either by displacing thediffraction grating on the object end of the imaging system involved orby displacing the mask on its object end. The lengths of travel employedin this procedure, which is termed “phase shifting,” are typicallyfractions of the grating periods involved. For practical reasons, in thecase of interferometers employed for measuring high-resolution,microlithographic reduction lenses, the grating on the latter's imageend bearing the reference pattern is usually translated, since both thelengths of travel on their object ends and the masses of the items thathave to be translated are greater.

[0007] The accuracies of these phase shifts significantly affectmeasurement accuracy and must be accurately controlled to within a fewnanometers in the case of applications in which spatial resolutions ofthe order of nanometers are to be assessed. Since a two-dimensionaldiffractive structure having several periodicity directions ispreferably employed, displacing the grating substrate along mutuallyorthogonal periodicity directions orthogonal to the optical axis of theimaging system involved is required. In order to determine the contrastof interference fringes along an imaging direction, the contrast ofinterference fringes along another imaging direction is reduced to zeroby a relatively rapid motion of the grating, with or without reversalsof its direction of motion. In the case of this oscillatory motion ofthe grating in the plane of the grating, any displacements of thegrating out of that plane are to be avoided at all times. These demandson the mechanism controlling the motions of the grating result in arelatively complicated design of that mechanism. Furthermore, reactionsdue to forces caused by accelerations may affect the entire setup andcause vibrations that will adversely affect metric accuracy.

[0008] Other interferometric devices for wavefront detection aredescribed in, for example, the article entitled “Phase measuringRonchi-test” by Omura, et al, that appeared in Applied Optics, Vol. 27,No. 3, pp. 523-528, German Patent Application DD 0 154 239, or GermanPatent Application DE 195 38 747.

[0009] Other testing methods, in particular, methods for measuring thedistortion of optical systems, are based on utilization of the moiréeffect. In the case of those methods, an object grating comprising, forexample, a large number of parallel, opaque, lines forming an objectpattern, is arranged in the object plane of the optical system to betested. An image pattern similar to that object pattern is arranged inits image plane, where the object pattern and image pattern are adaptedto suit one another such that a superposition pattern in the form of amoiré-fringe pattern is generated when the object pattern is imaged ontothe image pattern by the imaging system. Imaging parameters, inparticular, parameters indicating distortion generated by the imagingsystem, may be determined from the intensity distribution of this fringepattern. Moire methods are known from, for example, U.S. Pat. No.5,767,959, whose content is largely identical to that of U.S. Pat. No.5,973,773 or European Patent EP0418 054.

[0010] Separate light sources or illumination devices are provided forilluminating the respective semitransparent masks involved.

OBJECTS OF THE INVENTION

[0011] The invention is based on an object of providing a measuringmethod and an associated measuring system that will allow rapid,flexible measuring of the imaging fidelities of optical imaging systems.It is a further object of the invention to impose relaxed demands on themechanical configuration of the measuring system.

SUMMARY OF THE INVENTION

[0012] In order to address these and other objects, the inventionproposes a measuring method for measuring the imaging fidelity of anoptical imaging system having the following steps:

[0013] generating, in the vicinity of the object surface of the imagingsystem, at least one object pattern to be imaged, employing at least oneelectronically controllable pattern-generating device;

[0014] providing, in the vicinity of the image surface of the imagingsystem, a reference pattern adapted to suit the object pattern to beimaged;

[0015] superimposing an image of the object pattern on the referencepattern in order to generate a superposition pattern;

[0016] spatially resolved detection of that superposition pattern; and

[0017] determining at least one imaging parameter indicative of theimaging fidelity of the imaging system from that superposition pattern.

[0018] In keeping therewith, an associated measuring system formeasuring the imaging fidelity of an optical imaging system includes:

[0019] a device for generating at least one object pattern in thevicinity of the object surface of the imaging system, where this devicecomprises at least one electronically controllable pattern-generatingdevice and an electronic controller designed for controlling thatpattern-generating device;

[0020] a reference pattern adapted to suit that object pattern that isarranged on the image surface of the optical imaging system;

[0021] a detector for spatially resolved detection of a superpositionpattern; and

[0022] a device for determining at least one imaging parameterindicative of the imaging fidelity of the imaging system from thatsuperposition pattern.

[0023] Since, in the case of measurements on imaging systems, the lightpath involved is, in principle, reversible, any surface conjugate totheir object surface, for example, the surface that serves as theirimage surface when they are employed as intended, may be employed as theobject surface. In view thereof, according to the invention, thereference pattern may also be generated with the aid of anelectronically controllable pattern-generating device. Methods whereinboth the object pattern and the reference pattern are electronicallygenerated are also to be included thereunder. The object surface andimage surface may be planar, particularly in the case of photographicapplications or applications to photolithography. In the case of otherapplications, such as applications to movie projectors or planetariumprojectors, simply curved or compound-curved image and/or objectsurfaces are also encountered. Depending upon the particular applicationinvolved, the object pattern and/or reference pattern may be arrangedeither accurately on the object surface or reference surface,respectively, or in their vicinities, instead of on those surfaces.

[0024] The invention thus proposes providing that at least one of thepatterns to be superimposed on one another, in particular, the objectpattern, be electronically generated, rather than a rigid mask having apredetermined geometry and, consequentially, a predetermined imageablepattern. This replacement of at least one rigid, physical, mask, forexample, an illumination mask, by an electronically generatableequivalent in accordance with the invention allows dispensing with someor all rigid, physical, conventional types of masks.

[0025] Electronic generation of at least one pattern opens up newavenues when employed in combination with measurements on opticalcomponents. For example, the object pattern may be altered byelectronically controlling the pattern-generation device employed forelectronically generating a pattern that has been altered relative to abasic status, without need for employing any mechanically driven parts.For example, the alteration of the object pattern may comprise atranslation and/or rotation on an object surface, where the objectpattern as a whole may preferably remain essentially unaltered in termsof its shape and dimensions, i.e., for example, in terms of its gratingconstant(s) in the case of a linear grating or graticule, which willallow replacing mechanically driven motions of a mask by electronicallygenerated motions of an electronically generated pattern. Thisreplacement of mechanical mechanisms by electronics may, for example, bebeneficially employed for generating the phase shifts mentioned above byelectronically stepping a largely unaltered pattern along an imagedirection. Similarly, an inherently largely unaltered pattern may berotated about an axis oriented parallel to the optical axis of themeasuring system employed. Such rotations may, for example, bebeneficial in the case of the moiré method mentioned above in order todetermine distortion components along various image directions withoutneed for rotating the lens to be measured about its longitudinal axis.

[0026] Alterations of the object pattern (and/or reference pattern) thatwould be impossible employing conventional, rigid, masks due to theirnatures are also feasible. For example, the alteration of the objectpattern might comprise a stretching or compression of its basicarrangement, i.e., a change of spacing along at least one imagedirection. For example, a one-dimensional stretching of a linear gratingconsisting of parallel lines orthogonal to its lines may be utilized inorder to alter its grating constant. A corresponding effect ontwo-dimensional gratings, for example, graticules or checkerboardgratings, may be obtained by stretching/compressing their patterns alongtwo mutually orthogonal image directions. Radially symmetricstretchings/compressions of radially symmetric grating patterns, forexample, concentric circles, are also feasible.

[0027] The invention also allows a new type of measuring method, whichshall herein be termed the “radial-shearing method.” This method employsobject patterns that have a translation center within their patternedarea. In the case of phase shifts, the alteration of the object patterncomprises a translation of its patterned structures along radialdirections whose origins coincide with its translation center, where, inparticular, the periodicities of its patterned structures, i.e., theirgrating constants, may be held constant during that translation. Forexample, in the case of a radial-shearing interferometer, thediffraction grating involved may be rotationally symmetric, and, inparticular, may consist of an arrangement of concentric circular lines.The diffraction orders, or shearing directions, are orthogonal to thegrating lines, i.e., are radially disposed with respect to the center ofthe array of rings. If its grading period remains uniform, or is heldconstant, the diffraction angles along the radial direction will beconstant. The shearogram will thus yield the wavefront shifts along theradial direction. These radial phase shifts may be employed in eithershearing interferometry (interferometry employing a mask and adiffraction grating) or moiré methods (methods employing an object orimage grating). Radial phase shifts of radially symmetric, inparticular, rotationally symmetric, patterns orthogonal to grating linesare impossible if rigid, physical, conventional types of gratings areemployed.

[0028] In addition to rotationally symmetric object and/or referencepatterns, other radially symmetric patterns, for example, patternshaving multiple rotational symmetries, may also be generated andemployed. For example, gratings having threefold or fourfold symmetrymay be employed for measuring geometric shapes, such as pyramids,octahedra, or similar.

[0029] In general, the invention allows employing virtually arbitrarypattern geometries or grating geometries adapted to suit specialmeasurement tasks and conducting phase-shift methods employing thosegrating geometries. For example, object patterns that have at least oneperiodicity-direction gradient and/or a periodicity gradient withintheir patterned areas may be utilized. Such gradients allow havingseveral two-dimensional sections or zones having patterns with differingperiodicity directions and/or periodicities, where, in the case ofcontinually varying gradients, their periodicity directions andperiodicities may gradually merge into one another. Defining clearlydelimited areas or zones in order to allow making measurements on, forexample, multifocal lenses, is also feasible. In order to generate phaseshifts, the translations of the grating patterns may be performed alongdiffering translation directions or translation paths separately forseveral zones, since the invention allows totally independentlyutilizing zones within a patterned area having several zones.

[0030] A variation on the method provides that the alteration of theobject pattern comprises superimposing a prescribed intensity profile onthe object pattern. For example, that superposition might involvealtering a periodic object pattern having an essentially constantaverage image brightness over the entire image field such that the imagebrightness monotonically increases or decreases from the center of theimage field out to its perimeter. An intensity gradient along an imagedirection might also be generated, if necessary. Superimposing anintensity profile on the object pattern may be utilized as, for example,a substitute for conventional neutral-density filters in order to, forexample, adjust the luminous-intensity distribution incident on aspatially extended detector to suit its spatial responsivitycharacteristics.

[0031] Alterations of object patterns comprising superimposing aprescribed distortion profile on them in order to generate a slightlydistorted pattern from a geometrically ideal pattern are also feasible.This approach may be utilized to, e.g., generate a residual distortionin object patterns that compensates for corresponding distortion errorsin imaging optics that follow them in the optical train, on whose outputend a geometrically ideal pattern will then be generated.

[0032] All object patterns and variations on the method will also allowgenerating an object pattern that has an at least piecewise continuouslyvarying transmittance or reflectance along at least one direction lyingon the object surface in a simple manner. Object patterns or maskshaving gray scales in order to, for example, obtain a sinusoidalvariation of their transmittance or reflectance, are thus feasible.Variations of that type allow greater freedom in defining theircoherence function in order to, for example, optimize their low-orderinterference-fringe contrast and suppress higher orders that act assources of interference. In the case of conventional methods,fabricating masks having gray scales is time-consuming, since, forexample, dithering techniques will have to be employed in conjunctiontherewith.

[0033] A significant reduction in parts or component counts may beachieved in the case of a preferred embodiment by virtue of the factthat its pattern-generation device comprises a self-luminous,electronic, two-dimensionally configurable, illumination unit, whichallows measurement directions for which the light source involved isitself two-dimensionally configurable in a controlled manner. In thecase of an application in the visible spectral region, high-intensitycolor displays, digital projectors, plasma-discharge monitors, orsimilar, may be employed as illumination units and may serve ascomputer-controlled, configurable, self-luminous, light sources. Ofcourse, the same also holds true for monochromatic embodiments. Thesedisplays may be followed by one or more optical devices that project thepattern appearing on their self-luminous screen onto the object plane orobject surface of the imaging system to be measured in order to, forexample, adjust image magnification/demagnification and the pupillaryillumination level to suit the testing tasks involved, depending uponthe particular testing task involved and the spatial resolution of thedisplay employed. In the case of this latter embodiment, the functionsof the separate, conventional, light source and object mask are combinedin a single unit having a self-luminous illumination unit, in whichcase, no separate light sources will be needed.

[0034] Alternatively thereto, or in addition thereto, it will also befeasible to employ at least one partially transparent or partiallyreflecting mask having an electronically generatable and/or alterable,two-dimensional, opaque or reflective, mask pattern that assumes thetask of conventional, opaque, patterns on mask substrates whengenerating the object pattern. For example, employment of components,such as liquid-crystal arrays (LCD-units) or similar switchable elementsthat allow spatially resolved variations of their transmittance orreflectance, will be feasible. They might be used as substitutes forconventional types of transmitting or reflecting masks if the particularapplication involved will allow. Illumination of these electronicallycontrollable mask units might employ, for example, refractive ordiffractive elements, microlenses, fiberoptic lightguides, or similar.

[0035] An embodiment that is particularly well-suited to performingmeasurements on photographic lenses and other optical imaging systemsfor use in the visible spectral region provides for transmission oflight having differing wavelengths through the optical imaging systeminvolved and spatially resolved detection of several superpositionpatterns correlated to the various respective wavelengths involved,where light having differing wavelengths or comprising differing,narrow, wavelength ranges may be employed in sequence. However, it willbe preferable if several wavelengths or wavelength ranges that, e.g.,correspond to the primary colors red, green, and blue, aresimultaneously employed or are employed in parallel. Simultaneous,monochromatic, measurements may be performed in a simple manner byproviding that a polychromatic light source that is capable of emitting,preferably simultaneously emitting, light having differing wavelengthsor comprising differing wavelength ranges is provided on theillumination end. This polychromaticity may also be generated byemploying suitable filters. A spatially resolving detector that isresponsive to several wavelengths and might, for example, be a colorcamera having a suitable CCD-chip, is preferably provided on thedetection end. The preferably simultaneous utilization of severalwavelengths allows qualitatively assessing in a simple manner, imagingerrors (chromatic aberrations) of imaging optics that depend upon thewavelength of the light employed. Although two different wavelengths(wavelength ranges) might be sufficient here, employing more than two,in particular, three, different colors or wavelengths is preferable inorder to obtain reliable spectral data. Preferred application areas ofthe polychromatic measurements described here are to the fields ofmeasuring optics, e.g., photographic lenses, lenses employed for aerialsurveys, medical optics, or similar, intended for use in the visiblespectral region. Wavelengths (wavelength ranges) falling outside thevisible spectral region, e.g., wavelengths falling in the infrared (IR)or ultraviolet (UV) spectral regions, may also be employed.

[0036] The polychromatic measurements, in particular, interferometricmeasurements, discussed here may be beneficially employed in the case ofany measuring methods for measuring the imaging fidelities of opticalimaging systems, in particular, may also be employed in the case ofmeasuring methods that employ rigid, non-electronically alterable,conventional types of masks, regardless of the characteristics of theinvention.

[0037] The invention may be utilized for performing eithersingle-channel or multichannel measurements. In the case ofsingle-channel measurements, it may be provided that the object patternand/or image pattern may be translated along the surfaces on which theyare arranged using translation devices in order to allow performingmeasurements at various field points distributed over the entire imagefield in order to, in that manner, e.g., scan an entire image field.Multichannel measurements, in which several field points that are fromremote from one another may be simultaneously measured, may be readilyaccomplished using the invention by subdividing the object pattern intoa large number of adjacent pattern segments that normally will have thesame shape. The subdivision involved may be achieved by, for example,subdividing the surface of a monitor screen into identical subsectionscovering its entire area or by providing several monitor screens,projectors, or similar that may be simultaneously controlled, ifnecessary.

[0038] If the type of imaging system to be measured, the spaceavailable, and other limiting conditions will permit, it may bebeneficial if at least one secondary radiating surface situated in thebeam path, between a primary source of radiation and the imaging systemto be measured, is irradiated when generating the object pattern. Thesecondary radiating surface involved may, for example, be in the form ofa ground-glass plate, diffusor, and/or reflecting secondary radiatingsurface in the form of, for example, a projection screen. This secondaryradiating surface is preferably mounted in the vicinity of the objectplane. The shape (planar, simply curved, or compound curved) of itssurface may be adapted to suit the optics to be measured. For example,movie screens frequently have cylindrical shapes, since the associatedprojection optics have cylindrical image surfaces. This fact may betaken into account during measurements by providing a suitably curvedsecondary light source.

[0039] A variation on the method provides that, in order to generate theobject pattern, a secondary radiating surface having at least one lightray that may be reoriented relative to the secondary radiating surfacein a controlled manner is irradiated such that the object pattern iswritten within the duration of a write interval. In the case of thiswriting projection, a laser projector, for example, a laser projectorincorporating a scanning mirror, may be employed as the light source.The time required for “writing” a complete object pattern is preferablysynchronized to the exposure time of a camera provided on the detectionend. Either reflecting or transmitting secondary radiating surfaces maybe utilized, depending upon the setup involved.

[0040] Providing at least one secondary radiating surface, which may beeither planar or curved, for example, cylindrical or dome-shaped, on thedetector end is also feasible. A secondary radiating surface thatperforms a frequency conversion in order to, for example, allow makingbetter use of detector responsivity, may also be configured, ifnecessary. This secondary radiating surface may be arranged either at alarge distance from the reference pattern, or in its immediate vicinity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The aforementioned and other characteristics of the invention areas stated in the accompanying claims, description, and figures, wherethe individual characteristics involved may represent either themselvesalone or several such in the form of combinations of subsets thereofthat appear in an embodiment of the invention and have been implementedin other fields, as well as beneficial embodiments that may alone beinherently patentable. The accompanying figures depict:

[0042]FIG. 1 a first embodiment of a measuring system according to theinvention, wherein the pattern-generation device incorporates aprojector and an image guide is employed for coupling the object patterninto the imaging system to be tested;

[0043]FIG. 2 a second embodiment of a measuring system according to theinvention, wherein a secondary radiating surface in the form of aprojection screen that is irradiated by a projector is arranged in theobject plane of the imaging system to be tested;

[0044]FIG. 3 a third embodiment of a measuring system according to theinvention, wherein a deflecting mirror is arranged between a projectorand the imaging system to be tested;

[0045]FIG. 4 a fourth embodiment of a (multichannel) measuring systemaccording to the invention that images several object patterns directlyonto various field positions on a single monitor;

[0046]FIG. 5 a fifth embodiment of a measuring system according to theinvention for performing multichannel measurements, wherein separatemonitors that are arranged on a curved object surface of an imagingsystem are provided for various field points;

[0047]FIG. 6 a sixth embodiment of a measuring system according to theinvention having single-channel, polychromatic, illumination and adetector that is responsive to several wavelengths;

[0048]FIG. 7 a seventh embodiment of a measuring system according to theinvention having a pattern-generation device that has an electronicallycontrollable, partially transparent, mask unit;

[0049]FIG. 8 an embodiment of a measuring system according to theinvention that is configured for a writing projection of the objectpattern;

[0050]FIG. 9 an embodiment of a measuring system for performing a moiremethod, where a reference grating is arranged directly on a transparentcover plate of a CCD-chip;

[0051]FIG. 10 an embodiment of a measuring system for performing a moiremethod, where a secondary radiating surface is arranged in the immediatevicinity of the reference grating;

[0052]FIG. 11 an embodiment of a measuring system that is configured inthe form of a radial-shearing interferometer;

[0053]FIG. 12 a schematized representation of a rotationally symmetricobject pattern having concentric circles;

[0054]FIG. 13 a schematized representation of the radially orienteddiffraction orders associated with the pattern shown in FIG. 12 thatcorrespond to the shearing directions;

[0055]FIG. 14 an example of a radially symmetric object pattern havingfourfold symmetry;

[0056]FIG. 15 an example of an object pattern having two zones that maybe separately utilized for measuring a bifocal lens;

[0057]FIG. 16 an object pattern having a gradually varying periodicitydirection and periodicity for measuring progressive lenses; and

[0058]FIG. 17 a rotationally symmetric object pattern having asinusoidal transmittance function along the radial direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] A first embodiment of a measuring system 1 according to theinvention that may be employed for, for example, measuring photographiclenses, and operates in the same manner as a shearing interferometerwill now be discussed based on the schematized, longitudinallysectioned, drawing shown in FIG. 1. This measuring system comprises apattern-generation device having a color monitor 2 that, in the case ofother embodiments, may be replaced by projection optics, e.g., a beamer.The color monitor 2 may be regarded as a polychromatic,two-dimensionally configured or configurable, incoherent light sourcefor the measuring system. The color monitor is followed by imagingoptics 3 for reducing images displayed on the monitor, where theseimaging optics 3 are arranged such that the image displayed on themonitor falls on an entrance surface of a flexible image guide 4. Theexit end of this image guide 4 is arranged in the object plane 5 of theoptical imaging system that follows it in the optical train. The opticalsystem 6 to be tested is mounted on a test mount (not shown) that isinsulated against vibrations. The end of the image guide facing theoptical system 6 to be tested may be translated along the object plane(x-y plane) using a translation device (not shown), as well as along adirection orthogonal to the object plane, in order to allow preciselyarranging it in that plane.

[0060] The optical system 6 to be tested, whose optical axis 7 isaligned parallel to the optical axis of the measuring system andparallel to this z-direction, comprises several lenses, only two ofwhich are symbolically indicated, and images the image appearing on theexit end of the image guide 4 onto the image plane 8 of the opticalsystem to be tested. A diffraction grating 9, for example, acheckerboard grating, that serves as a reference pattern of themeasuring system is arranged in that image plane 8. A ground-glass plateor diffusor 10 that serves as a secondary radiating surface on thedetection end is arranged normal to the optical axis 7 at a distancefrom the diffraction grating 9 in order to make interference patternsoccurring behind the diffraction grating 9 viewable. This ground-glassplate 10 is arranged in the object plane of a color camera 11 thatallows spatially resolved, two-dimensional, detection of superpositionpatterns captured by the ground-glass plate 10. Detection ofinterferograms by the color camera may be spectrally selective forseveral wavelengths simultaneously. The detector 11 comprises aphotosensitive sensor surface 12, e.g., a CCD-chip, and an imagingsystem 13 that is arranged between the diffraction grating 9 and sensorsurface and images interferograms or shearograms appearing on theground-glass plate 10 onto the sensor surface 12. The detector 12 isconnected to a computer unit 14 on which suitable image-processingsoftware that configures the computer unit such that it may serve as adevice for determining at least one imaging parameter of the opticalsystem 6 to be tested from the detected superposition pattern(interferogram) is installed. The computer unit 14 also operates as acontrol computer for controlling the imaging color monitor 2 in orderto, for example, generate various types of two-dimensional objectpatterns on the monitor's surface.

[0061] Measurement of the imaging fidelity of the optical system 6 to betested and the associated determination of at least one imagingparameter, e.g., distortion, chromatic aberration, etc., indicative ofits imaging fidelity using the measuring device 1 may be performed asfollows: That portion of the computer unit 14 for controlling the colormonitor generates a suitable object pattern that may correspond to amask pattern on conventional, rigid, masks on the high-brightnessmonitor 2. The object pattern involved might, for example, be thatappearing on a perforated mask having a two-dimensional, symmetricallydistributed, array of holes. The shape, color, and location of that maskpattern are electronically generated and adapted to suit the type andperiod(s) of the pattern on the diffraction grating 9, duly allowing forthe demagnification of the reduction optics 3 that follows and themagnification/demagnification of the optical system 6 to be tested. Thepattern on the diffraction grating may, for example, be a checkerboardpattern or an array of lines.

[0062] For example, the location and/or orientation of the patternelectronically generated on the surface of the screen of the colormonitor may be altered be under software control. The reduction optics 3images the onscreen image onto the entrance surface of the flexibleimage guide 4. The object pattern then appears in the form of ademagnified image of the mask pattern at the exit end of the imageguide, which is situated in the object plane 5. This object pattern maybe regarded as an incoherent, radiating, object having an electronicallypredeterminable structure or as a wavefront source of theinterferometer. The exit end of the image guide may be brought up to theobject plane and, if necessary, translated over the object plane, usingthe x-y-z translation unit. The optical system 6 to be tested images theobject pattern onto its image plane 8, where the diffraction grating 9serving as a reference pattern is situated. When an image of the objectpattern is superimposed on this reference pattern in order to generate asuperposition pattern, the diffraction grating in the image planegenerates various diffraction orders whose coherent superpositionsgenerate an interferogram that is made viewable by the ground-glassplate 10 that follows. The contrast of the resultant interferencefringes is determined by the degree of spatial coherence existing in theplane of the grating, which is determined by the suitability of thelayouts of the (electronically generated) mask and diffraction grating,duly allowing for the magnifications/demagnifications involved. Theresultant interferograms are simultaneously and spectrally selectivelyrecorded by the detector 11.

[0063] On the measuring system 1, the exit end of the image guide 4 istranslated along the x-direction and/or y-direction by a translationdevice (not shown) in order to scan it over the image field of theoptical imaging system 6. The detector 11 is simultaneously scanned insynchronism therewith. The diffraction grating 9 may be mounted suchthat it may undergo oscillatory motions along the x-direction ory-direction in the image plane of the imaging system 6, driven by atranslation device (also not shown), in order to reduce the contrast ofthe sets of orthogonal of interference fringes generated by thediffraction grating 9.

[0064] The computer unit 14 is connected to the monitor 2 and detector11 by data lines and control lines and controls the electronicgeneration of the mask pattern on the color monitor, the recording ofimages, and the readout of the camera 11, as well as any oscillatorymotions of the diffraction grating that may be required. The computerunit 14 also computes and analyzes wavefronts detected by the detector.The phase-shift method, which is well-known to specialists in the fieldand thus will not be discussed in any detail here, is preferablyemployed for analyzing the resultant interferograms, which requires thatthe relative phases of the diffraction grating 9 and object pattern bealtered in a controlled manner. Their relative phases must be altered inseveral stages and the associated interferogram cached in the computerunit 14 upon conclusion of each stage. These interferograms are thenjointly analyzed.

[0065] A special feature of the setup proposed here is that these phaseshifts are introduced solely by altering the phase of the mask patterngenerated on the color monitor 2 by the computer, without need for anymechanical motions of any parts of the entire measuring system. Thisapproach has significant engineering advantages and cost benefits. Sinceno parts need to be moved during measurement procedures, expensivemechanisms for generating phase shifts and their associated controlelectronics may be eliminated. Since no moving parts are required forrecording measurement data, measurements are more reliable and moreaccurate. Furthermore, the screen of the color monitor 2 (or theprojection optics employed, in the case of other embodiments)simultaneously serves as a light source and a (variable) illuminationmask. Expenditures for separate light sources that would otherwise berequired and for fabricating conventional masks (both their substratesand their patterns) are thus eliminated. Also worthy of note is the highdegree of flexibility afforded by measuring systems according to theinvention. The shapes and dimensions of object patterns may be readilyadapted to suit the measurement tasks at hand, which allows testing awide variety of optical systems, without need for changing themechanical setup of the measuring system.

[0066] Employment of a suitable combination of a polychromatic lightsource, e.g., a color monitor, and a broadband detector allowssimultaneous conduct of test procedures at several differentwavelengths, for example, three wavelengths corresponding to the primarycolors of additive color mixing. Several “color channels” will then besimultaneously utilizable, where each color channel detects its own,associated, interferogram. This approach (cf. FIG. 6) allows determiningthe chromatic aberrations of imaging optics under test using a singletest procedure.

[0067] We shall now describe an example of a measurement in the visible(VIS) spectral region. Wavelengths from the infrared (IR) or ultraviolet(UV) spectral regions may also be exclusively utilized or additionallyutilized for making either monochromatic measurements or polychromaticmeasurements, i.e., measurements at several wavelengths or coveringseveral wavelength ranges. Devices, e.g., suitable coatings insertedinto the beam path, that perform frequency conversions may also beemployed in order to generate transitions between spectral regions, ifnecessary. For example, an IR-camera may be employed as detector if amonitor that emits in the VIS spectral region and a frequency-convertingsecondary radiating surface that absorbs VIS-light and emits in the IRspectral region are employed. Configurable light sources emitting in theIR or UV spectral regions may also be employed.

[0068] In the case of the following sample embodiments of measuringsystems according to the invention, for simplicity, items having thesame or equivalent functions as items appearing in FIG. 1 have beenassigned the same reference numbers.

[0069] The second embodiment of a measuring system 20 according to theinvention depicted in FIG. 2 has been optimized for detectingsuperposition patterns generated by an imaging system 21 to be testedthat has a longer object-end focal length than the imaging system 6depicted in FIG. 1. The former imaging system 21, might, for example, bea telescopic lens. The test setup on the detector end of the imagingsystem 21 to be tested is identical to that of FIG. 1. The object plane5 of the imaging system 21 to be tested, in this particular case, aplanar projection screen 22, is arranged on its opposite, object, end.The latter is part of the pattern-generation device, which, in thisparticular case as well, also comprises a color projector 23 aimed atthe projection screen 22. The former serves as a primary light sourceemployed for projecting the makeup of a two-dimensional object patternonto the planar projection screen 22. The pattern reflected there is theobject pattern, which is imaged onto the diffraction grating 9 situatedin the image plane 8 of the imaging system to be tested by the optics 21to be tested. In the case of other embodiments having image-endsecondary light sources, a ground-glass plate that is illuminated fromthe rear and utilized in transmission is provided instead of theprojection screen 22. Test setups having object-end secondary radiatingsurfaces in the form of, for example, projection screens or ground-glassplates, may, for example, be beneficially employed in combination withprojecting primary light sources in case where test objects having longfocal lengths or image planes situated large distances away are to bemeasured in cramped quarters. The measurement procedures involved andanalysis of the interferograms generated may proceed analogously to thecase for the method described in detail in conjunction with FIG. 1.

[0070] The measuring system 25 schematically depicted in FIG. 3represents a third embodiment of the invention. In this case, thepattern-generation device comprises a projector 20 controllable by thecontrol computer 14 as its primary, electronically configurable, lightsource. Light emitted by this light source is deflected toward theimaging optics 28 to be tested by a planar deflecting mirror 27 inclinedat an oblique angle. An intermediate image containing the objectpattern, which is then imaged onto the diffraction grating 9 situated inthe image plane 8 of the optics 28, is created in the object plane 5 ofthe imaging optics 28 to be tested. Detection and analysis of theinterferograms generated proceed analogously to the case for thoseembodiments described above. The object pattern may be electronically ormechanically translated over the object plane in order to allowconducting measurements at various field positions of the image field.

[0071] The measuring system 30 depicted in FIG. 4 represents a fourthembodiment of the invention and is configured for conductingmultichannel measurements on optics 31 to be tested. “Multichannelmeasurements” in the sense of the term used here shall mean thatmeasurements may be simultaneously conducted at several field positionsof the optics to be tested, where a distribution of field points thatwill allow a reliable measurement over the entire image field in asingle step should ideally be selected, in which case no translationdevices or other devices that allow scanning the image will be required.The measuring system 30 comprises a computer unit 14 that controls amonitor 32 having a large screen such that a preferably regulararrangement 36 of partial sections 37 that essentially cover the entirescreen, and each of which has the same, for example, rectangular, shape.These partial sections are controlled by the control device 14 such thateach of them has the same patterning, e.g., a regular, two-dimensional,grid composed of points. The surface of the screen 33 is arranged in theobject plane 5 of the optics 31 to be tested. This approach allows bothimaging the object pattern directly onto the image plane 8 of the optics31 to be tested and a compact setup, since no additional opticalcomponents need be provided between the self-luminous, configurable,light source 32 and the optics 31 to be tested. The object pattern thatthe optics 31 to be tested images onto its image plane 8 is incident ona diffraction grating 9 that is situated there. The interferencepatterns occurring behind the diffraction grating serving as a referencepattern fall directly, i.e., without any secondary radiating surfaceshaving been inserted into the optical path involved, onto a suitablydimensioned CCD-chip 34 of a color camera 35. That is, a compact setupinvolving a minimum of components (a diffraction grating and camera) hasalso been implemented on the image end of the system. The measurementsinvolved and analysis of the interferograms generated proceedanalogously to the case for those embodiments described above, exceptthat image information from each of several channels is acquired andanalyzed simultaneously, which allows conducting extremely rapidmeasurements covering the entire image field of optics to be tested anda measurement setup that is able to get by without any moving parts.

[0072] The measuring system 40 depicted in FIG. 5 represents a fifthembodiment of the invention. This particular system is configured forconducting multichannel measurements on a movie-projector lens 41 thatis configured for illuminating a cylindrical movie screen and thus has acylindrical image surface 42 and a planar object surface 8, on which thefilm to be projected runs when the projector is operated as intended.Measurement of the projection lens takes place in a direction oppositeto the normal direction of light propagation, namely, proceeding fromthe curved image surface 42 that serves as a cylindrical object surface42 during measurements. A large number of identical color monitors 44are arranged on the object surface such that the surfaces of theirscreens facing the lens 41 coincide with the cylindrical object surface42. These color monitors 44 are simultaneously controlled by thecomputer unit 14 such that they all display identical patterns, whichallows simultaneously conducting measurements at a large number of fieldpoints, as in the case of embodiment depicted in FIG. 4. Once again, acompact setup consisting of a diffraction grating 9 and a camera 35equipped with a large cameral chip only is provided on the image end ofthe system, as in the case of the embodiment depicted in FIG. 4. Anobject plane and/or image plane having curvatures other than thoseillustrated, for example, a dome-shaped image surface like thoseemployed on planetarium projectors, instead of the cylindrical andplanar surfaces shown, might also be taken into account. Projectionsystems, e.g., projection systems having point-source light sources andoptimized projection optics or laser projectors, might also be providedinstead of the monitors 44.

[0073] The measuring system 50 depicted in FIG. 6 represents a sixthembodiment of the invention with which single-channel, polychromatic,measurements may be conducted. Except for the configuration of itsdetector, the setup corresponds to that of FIG. 1, which is why thecomponents have been labeled with corresponding reference numbers. Thedetector 51 comprises a color camera equipped with three, spatiallyseparated, CCD-chips 52-54 and is preceded by a beamsplitting device 55.Each of these CCD-chips is configured for detecting one of the primarycolors, red, green, or blue, in order that three different wavelengths,or three different, relatively narrow, wavelengths ranges, may besimultaneously detected. This arrangement takes account of the fact thatseparate interferograms or superposition patterns that are independentof one another and may be separately detected and analyzed occur for thevarious wavelengths emitted by the color monitor 2. Employing thisapproach will allow rapidly, reliably, determining the chromaticaberrations of optics to be tested, where the results obtained for thesethree colors may serve as reference points when, for example, computinga dispersion curve.

[0074] Each of the sample embodiments discussed above based on FIGS. 1-6utilizes self-luminous, configurable, light sources, for example, lightsources having color or monochromatic monitor screens or projectors,where the functions of light source and illumination masks, which, inthe case of conventional measuring systems, are separate units, may becombined into a single unit. The seventh embodiment of the invention, tobe described below in terms of an example, based on FIG. 7, representsyet another opportunity for generating an object pattern to be imaged inthe vicinity of an object surface of an imaging system with the aid ofan electronically controllable pattern-generation device. In the case ofthe measuring system 60 depicted in FIG. 7, the pattern-generationdevice comprises a light source 61 whose light is directed toward anelectronically controllable mask unit 63 situated in the object plane 5of the imaging system 64 to be tested by the computer unit 14 with theaid of illumination optics 62 that follow the light source in theoptical train. The electronically controllable mask unit replaces therigid chromium masks usually employed on the illumination ends ofconventional measuring systems. This mask unit may have anelectronically controllable matrix structure whose transmittance, asindicated in FIG. 7, or reflectance may be altered. For example, itmight be configured in the form of a liquid-crystal array. Thiselectronically controllable mask unit may assume both the functions ofconventional transmission masks or reflection masks and the function of,e.g., phase shifting, in which the location and/or orientation of aninherently unaltered pattern is electronically shifted under softwarecontrol. Incorporated into the mask unit are, for example, elements,such as liquid-crystal arrays or similar switchable elements, whosereflectances or transmittances are variable under electronic control inspatially resolved manners. In view of the simple means for varyingobject patterns, the same benefits as for the aforementioned embodimentsapply, since the shapes and dimensions of the grating elements may beflexibly adapted to suit the particular measurement task involved. Needfor interchanging individual components (chromium masks), as well as theassociated need for realignment, are eliminated, as are expenditures forfabricating the components involved. Phase shifting or some other motionof the object pattern without any mechanical motions are realizable byshifting the electronically generated grating pattern, which allowsdispensing with expensive precision mechanics and their controlelectronics.

[0075] Another major advantage of electronically controllable masks orpatterns of the type described here is the opportunity for conductingautomatic calibrations and adjustments of measurement ranges, whichmight be accomplished by, for example, superimposing a suitableintensity profile on a basic pattern, i.e., distributing a tailoredintensity gradient over the image surface. Employing this approachallows adapting a detector-end characteristic curve, for example, thecharacteristic curve of the camera, including analog/digital converterthat provides digitized brightnesses of pixels and the characteristiccurve of the transmitter, i.e., the intensity profile of thelight-source end, to suit one another. This latter “transmittercharacteristic curve” is particularly strongly affected by thedigital/analog conversion on the light-source end, the brightnesses ofmonitor pixels, the need to take account of the spectral reflectance ortransmittance of a secondary radiator (projection screen, image guide,ground-glass plate), and the transmittance of the optics to be tested.In addition to the nonlinearities of analog/digital converters ordigital/analog converters, the spectral dependence of the responsivityof the detector or sensor surface must also be taken into account,particularly if intensity falloffs toward the perimeters of pupils,which are normally proportional to the cos4 of the angle of incidence,must be compensated by a tailored intensity profile, which will allowemulating a function corresponding to a neutral-density filter. Need foremploying separate neutral-density-filter units like those occasionallyemployed on conventional measuring devices is thus eliminated. Suitablytailoring the intensity profiles of electronically generated maskpatterns will thus allow linearizing transmission trains in a simplemanner, and maximizing modulation depth or contrast, combined with afavorable signal/noise ratio, on the detector end. Normalizing themeasurement range in order to account for under-riding limits andsaturation limits is also feasible.

[0076]FIG. 8 schematically depicts a measuring system 65 wherein asecondary radiating surface 66 formed by a projection screen isirradiated by a light beam 67 that may be moved relative to a secondaryradiation surface in a controlled manner such that it “writes” theobject pattern onto the secondary radiating surface within a writeinterval in order to generate the object pattern. A laser projector 68or similar writing light source is controlled by a computer unit 14 inorder to provide this writing projection. Control of the light beamproceeds at a rapid rate using, for example, an ultrafast scanner mirrorpresent in the projector 68 that rapidly shifts the position of thelight beam 67 on the projection screen 66. The computer unit 14 alsocontrols the camera 11 serving as detector, where the exposure time ofthe camera and the “write time” for the mask pattern are synchronizedsuch that the entire mask pattern will be “written” at least once withinthe exposure time. The write interval, within which the entire maskpattern must be covered by the light beam at least once, should thus beset less than, or equal to, the exposure time. A writing projectioninvolving several light beams, where each of the light beams writes onlyportions of the overall object pattern, if necessary, is also feasible.The remainder of the setup of the measuring system, complete with theoptics 6 to be tested, diffraction grating 9, and ground-glass plate 10,corresponds to the setup shown in FIG. 1 and described in detail inconjunction therewith.

[0077] The foregoing examples have been described based oninteiferometric measuring systems, where, in particular, a device fordetecting wavefronts according to German Patent DE 101 09 929 operatingin a manner similar to a shearing interferometer may be utilized. In thecase of this particular system, a reference pattern configured in theform of a diffraction grating is situated in the image plane of theimaging system. The superposition patterns (interferograms) resultingfrom superimposing waves generated by diffraction are detected andanalyzed. However, the invention may also be utilized in othermeasurement techniques, for example, measurement techniques that utilizethe moiré effect, in which case, bright-dark superposition patterns.generated by superimposing bright-dark patterns are primarily analyzed.An example of a moiré measuring system 70 for determining distortion ofoptics 71 to be tested is depicted in FIG. 9. The surface of the screenof a monitor 72 that is controlled by a computer 14 that also controls acamera 73 serving as detector and processes signals coming from thecamera is situated in the object plane 5 of the optics 71 to be tested.The camera 73 has a CCD-chip 74 whose sensor elements are covered andprotected by a transparent cover, for example, a cover glass 75, as itssensor surface. A reference pattern or image grating situated in theimage plane 8 of the optics 71 to be tested is applied directly on thiscover glass. The associated object pattern is electronically generatedby in the object plane 5 by the monitor 72. If the object pattern isimaged onto the image pattern by the optics 71 to be tested, then theCCD-chip will detect a two-dimensional intensity pattern that isgenerated by superimposing the imaged object pattern on the referencepattern and contains information on the imaging fidelity of the optics71 to be tested. Procedures for conducting such moiré methods andanalyzing the results obtained are well-known to specialists in thefield, and thus will not be discussed in any detail here.

[0078] The measuring system 80 depicted in FIG. 10 is also configuredfor distortion measurements employing moiré techniques and differs fromthe measuring system 70 depicted in FIG. 9 only in relation to theelements on its detector end. The reference pattern is situated in theimmediate vicinity of the planar image surface 8. A secondary radiatingsurface 81 that may be configured in the form of a scattering surfaceand/or a frequency-conversion surface is situated in the immediatevicinity of the reference pattern, in particular, directly on the imageplane or a short distance behind the image plane. Incorporating ascattering plate in this vicinity may destroy the spatial coherence andthereby suppress the Talbot effect. The Talbot effect generates aself-image of the reference grating at periodic distances behind thereference grating, and thus generates additional, “false,” sets of moiréfringes, and would thus falsify measurement results. The separation ofTalbot orders depends upon the grating period and the wavelengthinvolved. If a frequency conversion is required because, for example,the spectral responsivity range of the detector 11 has been poorlymatched to the spectral emission range of the structured light source72, that may be accomplished by, for example, by means of a thin,fluorescing, grating substrate on a thick, glass, carrier or by means ofa sandwich having a relatively thin, transparent, grating substrate onthe thick carrier substrate, with a fluorescing layer that may, forexample, contain an optical cement into which a fluorescing powder hasbeen admixed, sandwiched between them. In any event, intensitydistributions occurring in the vicinity of the image plane are imagedonto the sensor surface of the detector 11 with the aid of imagingoptics 82. Since the reference grating, together with a ground-glassplate or some other secondary surface, is situated in the image plane 8,the imaging optics 82 images the image field occurring there directlyonto the sensor surface of the camera.

[0079] We shall now discuss a new type of measuring method made possibleby the invention whose essential element is employment of radiallysymmetric patterns for which a phase shift along the radial direction ispossible, based on the measuring system 85 depicted in FIG. 11. Inparticular, its electronic pattern generation allows simultaneous,equidistant, phase shifts of radial gratings. Phase shifting insimultaneous, equidistant, steps simplify phase analyses. In particular,the method may be employed in the case of shearing interferometers andmoiré methods. Particularly useful in that conjunction is generation oremployment of rotationally symmetric patterns, which represent a specialcase of radially symmetric patterns, and, in particular, may be employedto major advantage in cases where rotationally symmetric errors are ofinterest or dominant. Such may be the case when testing asphericaloptics, where large dynamic ranges that may be obtained by adjusting thegrating period are required in the case of large asphericities.

[0080] The example depicted in FIG. 11 illustrates a prospective setupof a radial-shearing interferometer for wavefront measurements on asingle lens 86 that may be either a spherical lens or an asphericallens. The pattern-generation device comprises a monitor 2 that serves asthe two-dimensional, incoherent, light source of the measuring system,controlled by a computer 14. The image of the monitor is imaged onto theentrance surface of a flexible image guide 4 whose exit surface isarranged in the object plane 6 of the optics 86 to be tested by imagingoptics 3. The exit end of the image guide is translatable along bothaxes, normal to the optical axis. The diffraction grating 9 is situatedin the image plane 8 of the optics 86 to be tested. This grating is afollowed by a microscope objective that directs the wavefronts of thesuperposition pattern to the sensor surface 12 of the camera 11.

[0081] Interferometric testing of the transmittances of singlet lensesusually requires employment of compensating optics due to their largespherical aberrations, since the wavefront gradients cause veryspatially dense interference fringes that thus may no longer beresolvable by the detector. The purpose of the compensation optics iscompensating for the large gradients generated by the optics to betested in order that the remaining wavefront distortions will fallwithin the resolution range of the interferometer. Standard types ofinterferometers, such as Fizeau interferometers or Twyman-Greeninterferometers, may be employed if compensation optics are employed.However, compensation optics may be elaborate, since they normally haveto individually designed and calibrated for each optical system to betested. Compensation for rotationally symmetric aberrations is typical,since it may be accomplished by employing combinations of sphericalsinglet lenses.

[0082] A special feature here is that the monitor 2 generates arotationally symmetric object pattern consisting of concentric,circular, lines (cf. FIG. 12). The diffraction grating 9 has acorresponding, rotationally symmetric, pattern of concentric, circular,lines.

[0083] In the case of any measurement method that involves phaseshifting grating lines, the grating pattern will invariably have to betranslated normal to the grating lines, where the grating constant mustremain unchanged thereby. For example, in the case of thelateral-shearing interferometer mentioned above, a simple lineartranslation of the entire grating along a direction normal to thegrating lines is required. Correspondingly, a translation normal to thecircular grating lines, i.e., along the radial direction 89, referred tothe center of symmetry 90, which also represents the translation centerof phase shift, is also required in the case of a rotationally symmetricpattern. Such a radial translation, for which the grating period, i.e.,the radial spacing of the grating lines, must remain unchanged, is, inprinciple, impossible in the case of rigid gratings, e.g.,chromium-on-glass gratings. In the case of gratings on stretchablecarriers, although a stretching or compression is feasible, it wouldalter their grating constant. However, the invention allows phase shiftsalong the radial direction while leaving grating constants unchanged. Ashas been schematically indicated in FIG. 13, the diffraction orders areorthogonal to the grating lines, which also correspond to the shearingdirections or directions of the phase shifts involved. An unchangedgrating period during phase shifting causes the diffraction angle in theradial direction to remain constant, which greatly simplifies analysesof the resultant data. The interferograms (shearograms) obtainable usingthis method thus yield the wavefront displacements along the radialdirection.

[0084] The invention also allows adapting the geometry of the objectpattern to suit the measurement task involved in a simple manner. As anexample thereof, FIG. 14 depicts a radially symmetric object pattern 92having fourfold symmetry that is transformed into itself for every 90°rotation about its center of symmetry 93. This pattern has four mutuallyorthogonal grating directions along which equidistant, straight, gratinglines 94 lie. Associated with this pattern are two orthogonaldiffraction directions, indicated by the arrows. Patterns of this typemay, for example, be employed for testing pyramids, corner cubes, orobjects having similar geometries. The translation directions normal tothe grating lines, or along the direction of the diffraction orders, inthe quadrants may be either in the same direction or in oppositedirections. Radially symmetric patterns or gratings having other thanfourfold symmetry are also feasible. Careful selection of thetranslation directions will simplify correct determinations of algebraicsigns when analyzing measurement results.

[0085] In the case of patterns or gratings having radial symmetry,problems related to determining the algebraic signs of phases duringphase shifts may occur. In the case of a radial pattern, the phase ofthe grating changes along the radial direction, which means that thealgebraic sign reverses over its diameter, namely, at its center ortranslation center. The resultant phase thus also undergoes a change ofalgebraic sign that must be taken into account when computing phases.Since it may not be perfectly clear where the reversal of algebraic signmust occur, it will be beneficial if, in addition to a radial phaseshift, a lateral phase shift (linear phase shift) along a radius oralong several radii is also performed in order to check the algebraicsign and verify the consistency of measurements. Such phase shiftsinvolve displacing the entire pattern along one or more diameters byfractions of a grating period along lines passing through the symmetryorigin (translation center). The phase shift involved is thus in thesame direction over the entire diameter of the pattern and the locationwhere the reversal of algebraic sign occurs may be uniquely determined.If the lateral phase shift is performed along two directions that areinclined with respect to one another, the coordinates of the center,i.e., the coordinates of translation center, may also be uniquelydetermined.

[0086]FIGS. 15 and 16 depict two other options for adapting the patternperiodicities and periodicity direction or grating orientation of thegrating/mask combination to suit the measurement task involved. Theobject pattern 95 shown in FIG. 15 is intended for measuring optics tobe tested that have two focal lengths. It has two sections or zones 96,97 having rotationally symmetric patterns having differing gratingperiods, where the small, circular, zone 97 has a complete, rotationallysymmetric, pattern, and the large, essentially circular, zone 96 has anincomplete, rotationally symmetric, pattern having a larger gratingperiod. The phase shifts along radial directions toward the centers ofthe respective patterns may be performed separately for each of thezones. The locally alternating signals are analyzed in the mannerusually employed in the case of phase shifts.

[0087]FIG. 16 depicts an object pattern 100 that has a continuallychanging periodicity direction and periodicity, where both changes arelocally normal to the grating lines, within a circular patterned area,and shows the periodicity, or separations of the grating lines. Forexample, the spacing of the grating lines within the zone 101 enclosedby the dotted line is greater than that within the other zone 102enclosed by the dotted lines, and the periodicity directions 103, 104,which correspond to the shearing directions or translation directions,are inclined at an angle of about 20° with respect to one another. Agradual transition in both periodicity length and periodicity directionoccurs between these zones. The phase shifts for various zones, each ofwhich may be suitably spatially delimited, may be performed separatelyfor each zone in conjunction with measurements. Patterns of this typeare suitable for, for example, testing aspherical eyeglass lenses orprogressive lenses whose refractive powers gradually change over theirsurfaces.

[0088]FIG. 17 depicts an example of an, on the whole, circular,rotationally symmetric, object pattern 105 that demonstrates that,thanks to the invention, even nondigital masks or patterns may bereadily generated. Termed “nondigital” here are patterns whosetransmittance functions, or reflectance functions, can take on valuesother than 0 or 1, i.e., patterns having gray scales,. Patterns of thistype are usually rather difficult to generate employing, for example,dithering techniques. In the case of electronic pattern generation, onthe other hand, the control systems required are only slightly morecomplex than those required for generating digital patterns, such asthose depicted in, for example, FIGS. 14-16. This particular pattern 105has a sinusoidal transmittance function along the radial direction,i.e., a transmittance that gradually changes from values close to 0 andvalues close to 1 and back again along the radial direction. A mask ofthis type having a sinusoidal transmittance function optimizesinterference-fringe contrast for the zeroth and first diffractionorders. Fringe contrast for all other orders (interfering terms), on theother hand, is reduced to zero. “Holes” having a Gaussian profile aresuitable for suppressing interfering terms. Masks of this type havinggray scales thus simplify analysis of the results of measurements andmay yield more accurate measurements. Masks or patterns having grayscales extending over at least part of their patterned areas may beutilizable in conjunction with conventional patterns, for example,cross-hatched patterns, line gratings, checkerboard gratings, orsimilar, as well as those types of patterns that have been describedhere in terms of examples.

[0089] The above description of the preferred embodiments has been givenby way of example. From the disclosure given, those skilled in the artwill not only understand the present invention and its attendantadvantages, but will also find apparent various changes andmodifications to the structures and methods disclosed. It is sought,therefore, to cover all such changes and modifications as fall withinthe spirit and scope of the invention, as defined by the appendedclaims, and equivalents thereof.

In the claims
 1. Please cancel original claims 1-50 without prejudice ordisclaimer. Please add the following new claims:
 51. A measuring methodfor measuring imaging fidelity of an optical imaging system comprising:generating, in a vicinity of the object surface of the imaging system,at least one object pattern to be imaged, employing at least oneelectronically controllable pattern-generating device; providing, in avicinity of the image surface of the imaging system, a reference patternadapted to the object pattern to be imaged; superimposing an image ofthe object pattern with the reference pattern in order to generate asuperimposed pattern; detecting the superimposed pattern with spatialresolution; and determining at least one imaging parameter indicative ofthe imaging fidelity of the imaging system from the superimposedpattern.
 52. A method according to claim 51, further comprising:altering the object pattern by electronically controlling thepattern-generating device in order to electronically generate an objectpattern that is altered compared to a basic status.
 53. A methodaccording to claim 52, wherein no mechanical motions of any componentsare employed for altering the object pattern.
 54. A method according toclaim 51, further comprising altering the object pattern, said alteringcomprising translating the object pattern along the object surface. 55.A method according to claim 51, further comprising altering the objectpattern, said altering comprising rotating the object pattern within theobject surface.
 56. A method according to claim 51, further comprisingaltering the object pattern, said altering comprising stretching orcontracting a basic pattern.
 57. A method according to claim 51, furthercomprising altering the object pattern, wherein the object pattern has atranslation center and wherein said altering comprises translatingfeatures of the object pattern along directions that are radiallydisposed with respect to that translation center.
 58. A method accordingto claim 57, wherein a periodicity of the pattern features along thoseradial directions remains unaltered during that translation.
 59. Amethod according to claim 57, wherein said translating further comprisestranslating the pattern features laterally along at least one of theradial directions.
 60. A method according to claim 5 1, furthercomprising generating an object pattern that has at least one set offeatures that is radially symmetric with respect to a center of symmetryover at least some sections thereof.
 61. A method according to claim 60,wherein the object pattern is a rotationally symmetric object pattern.62. A method according to claim 51, further comprising generating atleast one of an object pattern that has a varying periodicity directionand an object pattern that has a varying feature periodicity within apatterned surface of the object pattern.
 63. A method according to claim62, wherein the object pattern is a continually varying object pattern.64. A method according to claim 51, further comprising generating anobject pattern which has a patterned surface having a plurality oftwo-dimensional sections that have at least one of differing featureperiodicities and differing periodicity directions.
 65. A methodaccording to claim 62, further comprising translating the object patterna plurality of times, wherein said translating of the pattern featurescomprises at least one of translating the pattern features separatelyalong differing translation directions and translating the patternfeatures around differing translation paths, for plural sectionsthereof, and wherein the sections have at least one of differingfeature-periodicities and differing periodicity directions.
 66. A methodaccording to claim 64, further comprising translating the object patterna plurality of times, wherein said translating of the pattern featurescomprises at least one of translating the pattern features separatelyalong differing translation directions and translating the patternfeatures around differing translation paths, for plural sectionsthereof, and wherein the sections have at least one of differingfeature-periodicities and differing periodicity directions.
 67. A methodaccording to claim 51, further comprising altering the object pattern,where said altering comprises superimposing a predetermined intensityprofile onto the object pattern.
 68. A method according to claim 51,further comprising altering the object pattern, where said alteringcomprises superimposing a predetermined distortion profile onto theobject pattern.
 69. A method according to claim 51, wherein the objectpattern is generated such that a transmittance or a reflectance of theobject pattern exhibits an at least piecewise-continuous variation alongat least one direction coinciding with a surface of the object pattern.70. A method according to claim 51, further comprising employing aself-luminous, electronically two-dimensionally structured illuminationunit for generating the object pattern.
 71. A method according to claim70, wherein the illumination unit comprises at least one of at least onemonitor and at least one projector.
 72. A method according to claim 51,further comprising employing at least one partially transparent maskhaving at least one of an electronically generated and an electronicallyaltered, two-dimensional, opaque mask structure for generating theobject pattern.
 73. A method according to claim 72, further comprisingemploying at least one electronically controlled component configured toprovide a spatial variation in transmittance of the component as themask.
 74. A method according to claim 73, wherein the component is aliquid-crystal array.
 75. A method according to claim 51, furthercomprising employing at least one partially reflective mask having atleast one of an electronically generated and an electronically altered,two-dimensional, reflective pattern for generating the object pattern.76. A method according to claim 75, further comprising employing atleast one electronically controlled component configured to provide aspatial variation in reflectance of the component as the mask.
 77. Amethod according to claim 76, wherein the component is a liquid-crystalarray.
 78. A method according to claim 51, further comprisingtransmitting light having a plurality of different wavelengths throughthe optical imaging system, and wherein said detecting comprisesdetecting, spatially resolved, plural superimposed patterns respectivelyassociated with the different wavelengths.
 79. A method according toclaim 78, wherein at least one of the imaging parameters is determinedas a function of the superimposed patterns detected at the differentwavelengths.
 80. A method according to claim 78, further comprisingusing a polychromatic light source designed to emit light having pluraldifferent wavelengths.
 81. A method according to claim 80, wherein thepolychromatic light source emits the plural different wavelengthssimultaneously.
 82. A method according to claim 78, further comprisingusing a spatially resolving detector responsive to the pluralwavelengths.
 83. A method according to claim 82, wherein the detectorcomprises at least one color camera.
 84. A method according to claim 78,wherein the light having the several different wavelengths istransmitted simultaneously through the imaging system and wherein thesuperimposed patterns associated with the different wavelengths aredetected simultaneously.
 85. A method according to claim 84, wherein atleast one of the imaging parameters is determined as a function of thesuperimposed patterns detected at the different wavelengths.
 86. Amethod according to claim 84, further comprising using a polychromaticlight source designed to emit light having plural different wavelengths.87. A method according to claim 86, wherein the polychromatic lightsource emits the plural different wavelengths simultaneously.
 88. Amethod according to claim 84, further comprising using a spatiallyresolving detector responsive to the plural wavelengths.
 89. A methodaccording to claim 88, wherein the detector comprises at least one colorcamera.
 90. A method according to claim 51, wherein visible light isemployed for generating the object pattern.
 91. A method according toclaim 51, wherein object patterns are generated at plural fieldlocations on the object surface of the imaging system.
 92. A methodaccording to claim 91, wherein at least one of the object patterns istranslated from one of the field locations to another of the fieldlocations between measurements in order to allow making measurements atthe plural field locations.
 93. A method according to claim 91, whereinsaid generating comprises generating plural object patternssimultaneously at the plural field locations on the object surface in amultichannel measurement operation.
 94. A method according to claim 51,wherein said generating comprises irradiating at least one secondaryradiator surface.
 95. A method according to claim 94, wherein thesecondary radiator surface is arranged in a vicinity of the objectsurface of the imaging system to be measured.
 96. A method according toclaim 51, wherein said generating comprises irradiating a secondaryradiator surface with at least one light ray that is shifted relative tothe secondary radiator surface in a controlled manner such that theobject pattern is written within a write-time interval.
 97. A measuringsystem for measuring imaging fidelity of an optical imaging system,comprising: a device generating at least one object pattern in avicinity of the object surface (5, 42) of the imaging system (6, 21, 28,31, 41, 64, 71, 86), said device comprising at least one electronicallycontrollable pattern-generation device and a controller (14) configuredcontrol the pattern-generation device (2, 23, 26, 33, 44, 63, 68, 72); areference pattern (9) adapted to the object pattern and arranged in avicinity of the image surface (8) of the optical imaging system; adetector (11, 35, 51, 73) for spatially resolved detection ofsuperimposed patterns; and a device (14) determining at least oneimaging parameter indicative of the imaging fidelity of the imagingsystem from the superimposed patterns.
 98. A measuring system accordingto claim 97, wherein the pattern-generation device comprises at leastone self-luminous electronically two-dimensionally structuredillumination unit.
 99. A measuring system according to claim 98, whereinthe illumination unit has at least one of at least one monitor (2, 32,44, 72) and at least one projector (23, 26, 68).
 100. A measuring systemaccording to claim 97, wherein the pattern-generation device comprisesat least one partially transparent mask (63) having a two-dimensional,opaque mask structure that is at least one of electronically generatedand electronically altered.
 101. A measuring system according to claim100, wherein the mask (63) comprises at least one liquid-crystal array.102. A measuring system according to claim 97, wherein thepattern-generation device comprises at least one partially reflectivemask having a two-dimensional, reflective mask structure that is atleast one of electronically generated and electronically altered.
 103. Ameasuring system according to claim 102, wherein the mask comprises atleast one liquid-crystal array.
 104. A measuring system according toclaim 97, wherein the object surface (42) has at least one of a one-axisand a multi-axis curvature.
 105. A measuring system according to claim97, further comprising at least one reflecting or transmitting secondaryradiator surface (22, 66) arranged in a vicinity of the object surface.106. A measuring system according to claim 97, wherein thepattern-generation device comprises at least one projector (68)generating at least one light ray directed at a secondary radiatorsurface (66) and scanned over the secondary radiator surface in acontrolled manner in a write-projector operation.
 107. A measuringsystem according to claim 97, wherein the pattern-generation devicecomprises at least one deflecting mirror (27).
 108. A measuring systemaccording to claim 97, wherein said detector is allocated to at leastone secondary radiator surface (10, 81).
 109. A measuring systemaccording to claim 108, wherein at least one secondary radiator surface(81) is arranged in an immediate vicinity of said reference pattern.110. A measuring system according to claim 97, wherein said detector(73) comprises at least one sensor surface covered by a transparentcover (75), and wherein the reference pattern is arranged directly onthe cover.
 111. A measuring system according to claim 110, wherein thesensor surface is a CCD-chip (74).
 112. A measuring system according toclaim 97, wherein said device for generating the object pattern isconfigured to operate at plural different wavelengths.
 113. A measuringsystem according to claim 112, wherein the plural different wavelengthsare generated simultaneously.
 114. A measuring system according to claim97, wherein said device for generating the object pattern comprises atleast one of at least one color monitor (2) and at least one colorprojector.
 115. A measuring system according to claim 97, wherein saiddetector (11, 5) is configured to detect superimposed patterns generatedusing plural different wavelengths.
 116. A measuring system according toclaim 115, wherein said detector comprises a color camera.
 117. Ameasuring system according to claim 97, configured to make measurementsat plural field points of the imaging system.
 118. A measuring systemaccording to claim 117, configured to simultaneously make measurementsat plural field points of the imaging system in a multichannelmeasurement.
 119. A measuring system according to claim 97, wherein thesystem has no mechanically driven moving parts.
 120. A measuring systemaccording to claim 97, wherein the system is configured as a shearinginterferometer.
 121. A measuring system according to claim 97, whereinthe system is configured to conduct a moiré technique.